In testing signal transmitting and receiving equipment which will be used over certain types of signal channels in which signal scattering and, hence, delays occur, it is desirable to use simulator equipment which will simulate as accurately as possible the characteristics of the signal channels involved. Thus, in wide band communication systems wherein the signal travels over the ocean, for example, the communication signal is often scattered back to the receiver with delays spread much more than the reciprocal of the signal bandwidth.
In simulating such delays, conventional channel simulators utilize a single, tapped delay line having a relatively large number of taps spaced therealong in accordance with conventional "Nyquist" spacing. In order to provide sufficiently accurate simulation in some applications, such a multi-tapped delay line simulator often requires more taps than is technically and economically feasible. For example, if a 50 microsecond delay spread occurs in the channel and the communication signal utilizes at 100 MHz bandwidth, such a Nyquist-spaced delay line would require 5000 taps therealong, with a Nyquist spacing of 10 nanoseconds. When such large bandwidths must be accommodated, it is not economically feasible to reproduce exactly the statistical characteristics of such large multipath delay spreads, such as can arise from ocean scattering, for example.
Conventional Nyquist spaced simulators as would be available for some applications are shown and described, for example, in the publication "Theory of a Tapped Delay Line Fading Simulator" by S. Stein, First IEEE Annual Communication Convention, Conference Record, Boulder, Colorado, June 7-9, 1965. The major cause for the high cost of such simulators lies in the fact that each tap is required to feed a signal to a multiplier and multiplied by a signal having Gaussian characteristics, such Gaussian signals being independent from each other at each tap. The large number of multipliers and the large number of independent Gaussian signal sources greatly increases the cost of the simulator for large bandwidth signals so that, in many applications, the overall costs become prohibitive and simplification thereof at the expense of accuracy becomes necessary.
In order to avoid excessive costs, it has been suggested that many of the Nyquist spaced taps simply be omitted to approximate as well as possible the channel characteristics, the accuracy of reproduction being traded off for such reduced cost factors. In such reduced-tap configurations, however, the energy concentrations at each tap are increased, since the total energy is utilized with fewer taps. Such a system would not provide realistic simulation under all conditions. For example, if, in an actual channel, the energy at a particular delay is lost, an actual receiver would simply move to an adjacent delayed signal during actual operation. In a simulator which uses an appropriate and relatively large number of taps at Nyquist spacing for simulating such a channel, if energy is lost at any one tap, the receiver would in a corresponding manner move to an adjacent Nyquist-spaced tap. However, in a simulator in which the relatively large number of taps is reduced considerably in order to avoid the high costs thereof, if a loss of energy occurs at any one of the taps, the receiver could not always move to an adjacent Nyquist-spaced tap because such adjacent tap may no longer be present in the simulator device. The simulation would thereupon become misleading and would incorrectly appear to simulate a situation in which the receiver is indicated as being non-operative when, in an actual situation, the receiver would be operative. Hence, the overall system's ability to simulate actual operating conditions of a signal channel is considerably reduced if the total number of taps at Nyquist spacing is reduced to a fewer number at spacings much larger than the normally required Nyquist spacing.